Age effects in animal models of abnormal behavior thought to be analogous to extrapyramidal and psychotic disorders are being investigated. Dose response functions for Ss of various ages have been established for psychomotor stimulant- and apomorphine-induced target behaviors (activity, stereotyped behavior, discriminative behavior). Work is being carried out to characterize the central locus of the age-related effects and to assess the effects of 5-HT and GABA manipulations on target behaviors induced by systemic drug administration. The most reliable findings to date indicate an absence of d-amphetamine augmented startle response in aged rats (24 months plus), and a decrease in stimulus properties of d-amphetamine by age 14 months.